The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals

A cyclic and an acyclic di(amino)carbene as well as a cyclic and an acyclic (alkyl)(amino)carbene cleanly react with benzoyl chloride to give the corresponding adducts 1⁺_cyc , 1⁺_acy , 2⁺_cyc , and 2⁺_acy , respectively. The reduction of 1⁺_cyc and 2⁺_cyc derived from cyclic carbenes affords the corresponding radicals 1_cyc and 2_cyc that are stable at room temperature. In contrast, radicals 1_acy and 2_acy , derived from acyclic carbenes, cannot be isolated. It is shown that 1_acy is as thermodynamically stabilized as its cyclic counterpart 1_cyc , but its instability is the result of β-hydrogens of the nitrogen substituent, along with the enhanced flexibility around C-N bonds, which allow for a H^. -migration-elimination process. Radical 2_acy is thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2⁺_acy and enolate 2^-_acy . This is due to the excessive steric hindrance, which prevents electron-delocalization on the NCCO fragment, and thus, the capto-dative stabilization. This work suggests general guidelines for the design of highly persistent (amino)(carboxy)radicals, especially by emphasizing the key advantage of cyclic patterns.